Computer disk drives store information on disks or platters. Typically, the information is stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information is written to and read from a storage surface(s) of a disk by a transducer. The transducer may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The transducer is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different data tracks. The disk is rotated by a spindle motor to allow the transducer to access different sectors within each track on the disk.
The actuator arm is coupled to a motor or actuator, such as a voice coil motor (VCM), to move the actuator arm such that the transducer moves radially over the disk. Operation of the VCM is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the transducer is moved from an initial position to a target track position. In general, the seek function is initiated when a host computer associated with the computer disk drive issues a command to read data from or write data to a target track on the disk. Once the transducer has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the servo control system is activated to center and maintain the transducer on the target track until the desired data transfers are completed.
The track following function of the servo control system generally includes maintaining the transducer at a desired position with respect to a track being followed (e.g., over a centerline of the track). Typically, the transducer must be moved slightly during track following to maintain a desired position over the track. This is because, due to various factors, the track may appear to move beneath the transducer. There are numerous outside influences which can make it difficult for the servo control system to maintain the desired position over the track, one such influence is known as “runout.”
Runout generally refers to deviation from perfect circular motion and, more particularly, refers to variation in the distance between an external point of reference and a passing surface of a rotating object. “Repeatable runout” involves periodic deviations that occur with predictable regularity (hereafter “RRO”). “Nonrepeatable runout” involves random perturbations due, for example, to bearing slop, shock events, and so on (hereafter NRRO). In the context of a disk drive, RRO is “repeatable” because it occurs in sync with the spinning disk. RRO may be caused by one or more of the following mechanical sources: a) spindle motor runout; b) disk slippage; c) disk warping; d) disturbances converted to RRO during the Servo Writing process due to, for example, NRRO, vibrations, resonances, media defects, or disk distortion due to clamping of the head disk assembly (HDA). RRO may also be caused by electromagnetic imperfections due to low quality servo information (i.e., servo bursts), even if they were mechanically recorded on the ideal circle. Such low quality servo bursts can yield incorrect position information.
The control system may compensate for RRO by sensing the RRO for a servo sector and then moving the transducer to follow expected movement of a track. For example, the control system may feed forward a combination of sinusoidal currents, which correspond to the harmonics of the RRO, and that are combined with a current command to the actuator in an attempt to cancel the RRO. Because RRO can change over time, the compensation can also be adapted based on newly sensed RRO for servo sectors. The RRO compensation should be based on the RRO that is sensed for all sectors around a disk. However, a transducer is often commanded to various different tracks before it can complete a full revolution on a track. Consequently, adaptation of the RRO compensation may be performed in a piece-wise fashion while the transducer is not seeking.
RRO compensation may become significantly distorted, yielding incorrect feed forward currents, if the adaptation is repeatedly performed based on RRO that is sensed for only a portion of a disk. To avoid such distortion, when adaptation is forced to stop, such as due to a seek command, adaptation may not be allowed to resume until the transducer reaches the servo sector that immediately follows the last servo sector for which adaptation has completed. In this manner, the adaptation can be based on all servo sectors of the disk. For high storage density disks that have a significant number of servo sectors, a high seek rate may cause a significant delay in adaptation of the RRO compensation, and which may significantly decrease the ability of the control system to maintain the transducer on track during track following.